US2272779A - Flash lamp - Google Patents

Description

Feb. 10, 1942. SARBEY' 2,272,779

FLASH LAMP Filed Dec. 27, 1939 l li I h'seconds 3mm Maurice D Sorbeg Patente Feb. 10, 1942 mesne assignments, to Hartford National vand Trust Company, Hartford, Conn, as trustee Application December 27, 1939, Serial No. 311,148

This invention relates to flash lamps, and more especially to novel alloys for use as combustible material in such lamps.

The flash lamp commonly used at this time consists of a transparent bulb containing a readily combustible material such as a metal wire or foil, 9. gas, which is usually oxygen. for supporting the combustion of the said material, and a means, usually electrical, for igniting the said material. When the igniter is energized, the very rapid combustion of the wire or foil gives off a single, very brief, and very intense light. The most important use of flash lamps is to illuminate objects to be photographed.

, Numerous devices are built, either separately, or

as integral parts of a camera, to synchronize the flash of the lamp with the operation of the camera shutter. It is, therefore, highly desirable that flash lamps shall have characteristics suited to this use.

In the making of flash lamps, the most generally useful combustible material is aluminum. However, aluminum is relatively hard to ignite and can be readily ignited only if it is reduced to a form having a very small cross-section. Thus if aluminum is used in the form of foil its thickness usually must not exceed about .00004 inch, while if used in the form of wire its diameter usually must not exceed a few ten thousandths of an inch. However, when it is reduced to such tenuous form its rate of combustion becomes too great to make a good lamp. A study of the variation of light intensity with time in such a lamp shows that the light in tensity climbs very rapidly to its maximum value then falls very rapidly again to practically zero. The result is that the major part of the light output of the lamp occurs in a. very few milliseconds, usually less than fifteen in certain com-' mon types of lamps. When such a lamp is used with a camera, almost its entire light output may be given ofl? either before the shutter opens or after it has closed, in spite of any but the most accurate synchronization, and if a focal plane shutter be used the light may not last during the entire time of sweep of the camera shutter across the plate or film, or may vary in intensity to an objectionable degree during such sweep, irrespective of even perfect synchronization.

For ease of synchronization and the best use with all types of cameras it is. therefore, desirable that a flash lamp shall emit the major portion of its light outpukrather evenly over a considerable duration of time, say 25 to 30 milliseconds for the most common use. It is also desirable that the intensity of light emission from the lamp shall attain a major portion of its maximum rather promptly after the lamp ignition circuit is closed, say in 12 to 15 milliseconds for most purposes. With pure aluminum these two requirements are to a large extent mutually exclusive, and any wire or foil so thin as to cause the metal to ignite quickly enough, in general burns too rapidly for, best results, while heavier wires or foils may have sufilciently long burning period, but if ignited at all, ignite too slowly for best results.

. The general object of this invention is, therefore, to provide aluminum alloys for use in flash lamps which so modify the aluminum as to preserve its advantages, and at the same time overcome its shortcomings.

A specific object of the invention is to provide aluminum alloys for use in flash lamps, which are more easily ignltible than aluminum.

Another object is to provide aluminum alloys for use in flash lamps, which, being more easily igniter is energized, and rather evenly for a rela tively long period thereafter.

Other objects of the invention will be apparent from the following specification.

In the accompanying drawing forming a part of this specification,

Figure 1 is a view of a flash lamp of the kind to which this invention relates, and

Fig. 2 is a diagram illustrating the illumination resulting from the use of different combustible materials in such a flash lamp.

The flash lamp shown in Fig. 1 is of a known type except as to the material employed in the combustible wire and the details are illustrated merely to give the setting of the use of the improved combustible material and, of course, may be varied as desired without affecting the invention.

In the construction shown, there is a filament I upon which there are heads 2 of igniter ma:- terial which may be of powdered zirconium or other similarly ignitible material. The filament is shown attached to lead wires 3-,3 which are mounted on a glass stem l within the transparent bulb 5, the bulb having a base 6 adapted to screw into a standard socket by which the bulb is con-v nected to a suitable source of electricity. Preferably there is an asbestos disk 1 about the tube 4 which prevents the ignitible wire 8 from entering the neck of the bulb. The space 9 about the wire contains oxygen.

It will be readily understood that when subjected to an electric current, the filament i becomes heated and ignites the beads 2 which in turn ignite the wire 8, the burning of which emits the desired light. The present invention relates to the material employed in forming the combustible wire 8.

The preferred alloy for wire 8 consists predominantly of aluminum with minor additions .of

one or more of the metals of the group consistin of zirconium, barium, strontium, cerium, and

misch-metal. The addition to aluminum of more than .5% .of the alloying metals improves the result, and as much as 1% of the alloying metals lumens and then rapidly decreased, being below 1,500,000 lumens at about 28 milliseconds.

produces a marked increase in the ignitibfllt of the resulting alloy when worked into fine wire or foil, as compared with aluminum. For most purposes, however, about 1% to 5% of the alloying metal and the balance aluminum produces a very satisfactory alloy. As the 'addition'of the alloying metals exceeds 5% the alloys become more difiicult to work. With careful working,

small percentages of reduction at each pass through the rolls, swagers, wire drawing dies, ,or other mechanical working equipment, and frequent intermediate annealings, alloys containing up to 15% of the alloying elements can be worked.

In the diagram constituting Fig. 2, there are indicated the performance curves of bulbs of the kind described above with combustible wires of three difierent compositions or sizes.

In the diagram, the ordinates show the approximate radiation in lumens, while the abscissas show time inmilliseconds.

It will be noted that when the improved alloy described in this application was employed for wire 8, the ignition was substantially as prompt,

the illumination of 1,500,000 lumens being reached at substantially the same time as in curve B, but slightly sooner if anything, and that the peak illumination was maintained nearly constant for a quite appreciable length of time, dropping to 1,500,000 lumens at about 43 milliseconds.

These performance curves clearly show that while the larger aluminum wire continues to emit as much as 1,500,000 lumens for an appreciable interval, it is somewhat slow in starting, the

period from the time of electrical contact until scribed in this application, the illumination begins as promptly as with the, small aluminum wire'and continues even beyond that of the aluminum wire of the same size.

Incidentally it may be pointed out that with the type of alloy disclosed in this application, the illumination not onlylstarts promptly and lasts during the effective range for a considerable time, but'also the combustion of the wire both before and after efiective degree of illumination results is comparatively short, whereas with the aluminum wire, the combustion before effective illumination is reached is somewhat longer, and after effective illumination ceases,

combustion tails out ,for a considerable time. This shows graphically the greater efliciency of the alloy disclosed in this application, since most of its illumination takes place through the effective range.

Curve A shows the perforance of a lamp in which wire 8 consists" of a substantially pure aluminum wire .001 inch in diameter. Curve B shows the performance of a flash lamp wherein the wire 8 consists of a substantially purealuminum wire about .0006 inchin diameter. C shows the performance of a lamp wherein the wire -8 is .001 inch in diameter and consists of an alloy containing 3.1% misch-metal, the balance being substantially all aluminum.

It will be noted that curveA indicates somewhat slower ignition than the other curves and after the wire begins to burn, there is a noticeably slower increase in illumination. For most purposes for which flash lamps of this type are employed, a radiation of 1,500,000 lumens may be considered satisfactorily effective. Taking this point for comparison, it will be seen that after making electrical contact, which time is indicated by the point zero, nearly 20 milliseconds elapse before the illumination by wire 8 reaches 1,500,000 lumens. After this point is reached the illumination gradually increases and then decreases until it drops below 1,500,000 lumens at about 41 milliseconds after the closing of the circuit.

In order to obtain a more prompt production of light, a smaller wire of the same composition was employed, and it will be seen from curve B that this resulted in a more prompt ignition and I the reaching of 1,500,000 lumens in approximately 14 milliseconds. The illumination rapidly increased from this point to a maximum of 2,000,000

Curve iii) 'Another important feature, especially wherea camera of the focal plane type is employed is the nature of the curve during the effective 11- lumination period. If instead of 1,500,000 lumens being taken as the point of comparison, we take 1,700,000, or slightly below half way between the 1,500,000 and the 2,000,000 lumens, it

will be seen that the curves B and C each reach this point at slightly before 13 milliseconds, but while curve B drops below this point atabout 25 milliseconds curve C continues at about this degree of radiation until about -36 milliseconds. Curve A does not reach this point until about 23 milliseconds. Curve C also drops below this point at about- 36 milliseconds. Therefore, the illumination period of curve C at 1,700,000

lumens is even longer in comparison with the other curves than at the lower range, Also, it will be seen that even during the. short period .when curves A and B are above 1,700,000 lumens their illumination varies much more rapidly thanvrith curve C. As a matter of fact, the variation in illumination in curve C is no greater at the top of the curve for 23 milliseconds than it is forabout -milliseconds in curve B or 8 mlh liseconds in curve A.

While curve C is as described forone specific alloy, it will be readily understood that other alloys coming within the range of this specification show an effect varying from pure aluminumin the same direction as curve C varies from curve A or B, although the degree of such be rolled or swaged, and then drawn into fine alloy, but not so long or even a burning period.

A similar barium alloy is slightly inferior to strontium, and an alloy with about zirconium is slightly inferior to barium. All of these alloys, however, are considerably superior to pure aluminum, and their curves, if shown on Fig. 2, would occupy positions intermediate between curves A and C.

The alloying elements above mentioned are highly and readily oxidizable materials, but not all such materials may be used. Thus calcium and lithium when added to aluminum even in very small percentageapmduce alloys that are so diflicult to work into fine wires or foils as to be useless iorpra'ctical purposes. Beryllium on the other hand produces workable alloys with aluminum, but does notmaterially improve its ignitibility.

In the making of the alloys of the present invention the aluminum is melted first, in any suitable funace, and brought to a temperature of about 800 C. The alloying element or' elements are then dissolved in it. Alloys of barium and strontium may be made by adding the pure metal, plunging it below the surface of the wires. The mechanical working can all be done cold, with occasional annealings as the metal becomes work hardened.

In the making of flash lamps it is now recog nlzed tht wire is usually preferable to foil, both because of the superior time-character of the dash produced by wire lamps, and because wires are easier to make and to handle. The alloys of the present invention are particularly suitable for making wire. In the common flash lamp wire of a diameter of about .001 inch is usually required. Because of the limited tensile strength of many aluminum alloys, wires of this small size aluminum by means ofa perforated iron cup,

where it quickly dissolves, but produces considerable oxidation and slagglng of the alloying material and a major portion of it may be lost. Another method is to maintain an atmosphere of helium or other inert gas in the crucible, which reduces the loss of strontium or barium by oxidation Still another method is to gener-- ate the metal in place by adding to the melted aluminum briquettes of a mixture of powdered aluminum with barium oxide or strontium oxide. This requires higher temperatures.

In the case of zirconium, this metal is commercially available as an aluminum alloy of relatively ,high zirconium content. The desired alloy'is, therefore, most easily made by adding this zirconium rich-alloy to a calculated amount of aluminum.

In the case of cerium and misch-metal, the metal is merely added to the melted aluminum, and because oi its high density it immediately sinks and then dissolves slowly with some oxidation. Maintaining the temperature and stirring occasionally until solution is complete finishes the process.

Misch-metal is the commercial name given to the naturally occurring mixture of the rare earth metals. These metals occur together in their ore (usually monazite sand) in fairly definite proportions. They are dimcult to separate from each other and are, therefore, commonly reduced as a group. This group usually contains about 50% cerium, 20% lanthanum, the balance being divided among the other rare earth metals of the group. Cerium is diihcult to make andis not available commercially. Misch-metal on the other hand is relatively inexpensive and is made commercially in considerable quantity for use in pyrophoric alloys. For this reason, and because it makes a more workable alloy with aluminum than does pure'cerium, it is preferred to pure cerium itself for the purposes of this invention.

After the alloys have been made, they ,are

are too dimcult, if not impossible, to draw directly. For greater convenience and economy the alloys of the present invention are, therefore, worked into'rods of about A inch diameter, which are then snugly fitted into copper tubes. These filled tubes are then worked into wires by the usual commercial methods used with copper, and with scarcely any more care or difficulty than is required for pure copper. In this way the copper furnishes the strength and support required to pull the aluminum alloy through the dies. When the filled tubes have been drawn down until the aluminum alloy core is of the desired size, the copper is dissolved ofi with a suitable difi'erential solvent that attacks copper but not aluminum, such ,as nitric acid for instance, or the copper can ,be removed by de- I plating.

In the making of wire filled flash lamps, it is often desirable that the wire shall have a certain stillness, or springiness. This makes for greater ease in handling and prevents it from matting together. i

In certain compositions of the alloys of the present invention, especially those in which the cast into the form of rods or ingots which pan percentage of the alloying element is very low, the natural springiness of the wire may be insufflcient for some purposes. In such cases it may be increased by adding a minor percentage of any one of a number of metals which are known to have a hardening effect on aluminum. Many such hardening agents are known in the metallurgyof aluminum, and among them are copper, silver, iron, nickel, cobalt, etc. The amount of hardening agent depends on the one used, but it is usually from a fraction of one percent to not over two percent.

What I claim is:

1. In a flash lamp a combustible element in the form of a long thin body, said element consisting of an alloy comprising .5% to 15% of a metal from the group consisting of zirconium, barium, strontium, cerium and misch-metal and the balance of said alloy being substantially all aluminum.

2, In a flash lamp a combustible element in the form of a wire, said element consisting of an alloy comprising .5% to15% of a metal from the group consisting of zirconium, barium, strontium, cerium and misch-metal, not over 2% of a metal having a hardening eflect on aluminum and the balance of the alloy being substantially all aluminum.

3. In a flash lamp comprising a transparent bulb, a wire within the bulb of an alloy of aluminum with from .5% to 15% of a metal of the group consisting of zirconium, barium, strontium, cerium and mis'ch-metal and with from a fraction of one percent to not over 2% of a metal having a hardening eifect upon aluminum, oxygen within the bulb, and means to ignite the wire. 7

4. In a. fiash lamp comprising a bulb, a combustible element in the form of a wire within the bulb, said element consisting of an alloy or aluminum with from 1% to 5% of a metal of the group consisting of zirconium, barium, strontium, cerium, and misch-metal and with not over 2% of a metal having a hardening effect upon aluminum, oxygen within the bulb 10 and means to ignite the wire element.

5. In a flash lamp a combustible element in ,the form of a long thin body, said element consisting of an alloy comprising 1% to 5% of a metal from the group consisting of zirconium,

barium, strontium, cerium and misch-metal and the balance of said alloy being substantially all aluminum.

MAURICE D. SARBEY.

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